Been in this fandom for a while and I keep noticing a trend with these two, especially in the darker, more character-driven stories. It's almost never a fluffy romance. The main thing that hooks writers is this idea of them as two apex predators recognizing each other, a mutual obsession that's more about power dynamics and psychological games than love. A lot of fics frame them as these inevitable, opposing forces—Illumi as control and cold calculation, Hisoka as chaos and visceral thrill-seeking. Their 'relationship' becomes a battle of wills, a high-stakes game where the question isn't 'will they kiss' but 'who's manipulating whom' and 'will one of them finally snap and try to kill the other for real this time'. It's super compelling because it respects how dangerous both characters are; they don't get domesticated.
You also see a ton of exploration around their canonical indifference to conventional morality, refracted through their dynamic. Like, Hisoka's turn-on for potential and Illumi's transactional view of human life create this weird, amoral intimacy. They understand each other on a level 'normal' people couldn't. I've read fics where Illumi analyzes Hisoka's Nen like a puzzle, and Hisoka treats Illumi's emotional vacancy as the ultimate challenge. The tension is less sexual (though that's definitely there) and more about this terrifying, fascinating symbiosis. It makes for a great read when you're in the mood for something intense and psychologically sharp, not sweet.